In devices that rely upon magnetic resonance, a large direct current magnetic field is applied and alternating current is applied for excitation. Sensitivity limits are well-recognized in magnetic resonance devices. Sensitivity limits result from the low signal to noise ratio in magnetic resonance systems. Prior techniques to increase the signal to noise ratio include the use of ever higher magnetic fields, better amplifier technology, and application of more efficient pulse sequences and signal processing techniques, among others.
Other research efforts have sought to change the basic model of inductive detection used in magnetic resonance devices. Alternate detection techniques that have been researched include, for example, force detection, direct transfer of angular momentum, and energy from the spin population in magnetic resonance using micro-mechanical cantilevers. Additional research has been conducted on the flux-detection class of magnetic resonance sensing schemes such as superconducting quantum interference devices, Hall sensors, and superconducting resonators, as well as optical methods.
Despite the ongoing research, the inductive coil has remained the main workhorse in commercial magnetic resonance systems, both in spectroscopic and imaging settings. One reason for the continued used of inductive coils is that the long-term development of the inductive coil technology successfully kept pace with the improved magnet designs. Another important reason is the versatility of inductive coils as current carrying structures and their utility in providing most of the critical features in a magnetic resonance instrument. Specifically, the inductive coils in magnetic resonance systems serve the dual important functions of providing both the AC magnetic field to excite a sample and detecting the signal from a sample.
In the absence of the large DC magnetic fields of magnetic resonance systems, magnetic amplification has been obtained previously. Significant amplification of the magnetic field produced by a solenoid can be achieved if a soft ferromagnetic material, such as iron, is inserted into the coil structure. However, the ferromagnetic material saturates in the large DC magnetic fields of magnetic resonance systems. Accordingly, research directed toward increasing sensitivity in magnetic resonance systems has looked to other approaches, such as the alternative detection techniques discussed above.
Particular magnetic resonance systems that would benefit from increased sensitivity include, for example, magnetic resonance imaging systems and magnetic resonance spectroscopic systems. Magnetic resonance imaging systems are the primary diagnostic tool in medicine for high resolution imaging of patients. Magnetic resonance spectroscopic techniques are invaluable in analytical chemistry, biology, and materials science. Despite the successful use of magnetic resonance, both spectroscopic and imaging applications of magnetic resonance have low signal-to-noise ratios due to the weak nuclear magnetic moment of the proton and low fractional polarization, even in large magnetic fields, at room temperature.
When the size of a magnetic particle is small enough, usually bellow 100(nm), it becomes energetically unfavorable for the magnetic domain walls to form within the nanoparticles, and the magnetization within them will be uniform. These nanoparticles usually have uniaxial magnetic anisotropy. This regime of nanoparticle magnetism is also called a single domain state, and has been studied by others. The principle is used, for example, in the manufacture of magnetic and optical recording media. The general physics of magnetization processes in single domain magnetic nanoparticles has been extensively studied. Particularly, the reversible transverse susceptibility, χRT, of single domain nanoparticles has also been studied since the features in χRT often reveal fundamental magnetic anisotropy information.